The present invention relates to an acceleration sensor for detecting an acceleration, and more particularly to an acceleration sensor for detecting an acceleration by sliding vibration, which is exerted when an acceleration is applied, and to an acceleration sensor device constructed by storing this acceleration sensor in a package.
An acceleration sensor is installed in equipment to monitor an abnormal condition of the equipment by detecting an acceleration and vibration of the equipment. For example, the acceleration sensor is used to prevent errors in reading and writing data resulting from vibration and shock in a hard disk drive, to prevent hand shaking in a video camera, to actuate an air bag in a vehicle, etc.
With a reduction in size and an increase in the performance of equipment in which an acceleration sensor is to be installed, there has been a demand for the development of a small-sized, high-performance acceleration sensor capable of being mounted on a surface of the equipment. As such a small-sized acceleration sensor, an acceleration sensor using a piezoelectric element has been conventionally put into practice. Disclosed examples of such an acceleration sensor include an acceleration sensor which detects an acceleration by using a deflection of a piezoelectric single crystal (Japanese Patent Application Laid-Open Nos. 10-206456/1998 and 11-211748/1999) and an acceleration sensor which detects an acceleration by using a deflection of piezoelectric ceramic (Japanese Patent Application Laid-Open No. 6-273439/1994).
Acceleration sensors using the deflection of a piezoelectric single crystal or the deflection of piezoelectric ceramic as mentioned above can improve the detection sensitivity by increasing the deflection to increase the stress. Thus, in order to improve the detection sensitivity for high performance, the mass needs to be increased to produce a larger deflection, causing a problem that the acceleration sensor becomes heavier and larger in size. On the other hand, when the piezoelectric element is made thicker, it does not easily deflect and causes a problem that the detection sensitivity is lowered. Then, in order to improve the detection sensitivity, there have been proposals to make the piezoelectric element thinner, to stick two pieces of extremely thin piezoelectric elements together, etc. However, such proposals are associated with problems that the fabrication process is complicated and the cost is increased.
Therefore, the applicant of the present invention proposed an acceleration sensor capable of detecting an acceleration with good sensitivity by a small-sized structure (Japanese Patent Application Laid-Open No. 2000-97707). This acceleration sensor comprises a vibrator and a weight section which is connected and supported at a position different from the position of the center of gravity of an assembly of the vibrator and weight section, and obtains the magnitude of an applied acceleration by detecting the amount of characteristic (sliding vibration) of the vibrator corresponding to an angular moment exerted in the weight section when the acceleration is applied. The present inventors are pursuing the development and improvement of such an acceleration sensor that has a small-sized structure and a high detection sensitivity without increasing the size of the vibrator to achieve high performance because it detects sliding vibration instead of deflective vibration.
The present invention has been achieved in view of the above-mentioned circumstance, and its object is to provide an acceleration sensor which is an improvement of the acceleration sensor proposed in Japanese Patent Application Laid-Open No. 2000-97707 and capable of realizing a further reduction in size and cost while maintaining high performance.
Another object of the present invention is to provide an acceleration sensor which does not limit sliding vibration (or, shearing strain) of a vibrator for the purpose of preventing a lowering of the detection performance.
Still another object of the present invention is to provide an acceleration sensor capable of improving the connectivity between a vibrator and a weight section.
Yet another object of the present invention is to provide an acceleration sensor device having a package structure capable of storing such an acceleration sensor efficiently without deteriorating its detection performance.
An acceleration sensor of the first aspect of the present invention comprises a vibrator provided with an electrode and subject to a sliding vibration; and a weight section connected to the vibrator and supported at a position different from the position of the center of gravity of an assembly of the vibrator and weight section, wherein the weight section is provided with a wiring pattern connected to the electrode, the vibrator detects an angular moment about the support point as sliding vibration, which is exerted in the weight section when an acceleration is applied, and an electrical signal corresponding to the applied acceleration is output from the electrode through the wiring pattern. In this structure, the weight section is provided with the wiring pattern for drawing an electrical signal corresponding to an acceleration to be detected, and also functions as a signal detecting substrate. Therefore, it is not necessary to newly provide a signal detecting substrate, resulting in a simplified structure and a reduction in the cost.
In an acceleration sensor of the second aspect of the present invention, a detection portion for sliding vibration of the vibrator is divided by electrodes which are a plurality of divided electrode parts. Since the detection portion for sliding vibration of the vibrator is divided into a plurality of parts by the electrodes, the detection section of the vibrator can be divided simply, thereby achieving a reduction in the cost.
In an acceleration sensor of the third aspect of the present invention, the detection portion for sliding vibration of the vibrator is divided by the electrodes which are a plurality of divided electrode parts and by a groove connected to the electrodes. Since the detection portion for sliding vibration of the vibrator is divided into a plurality of parts by the formation of the groove, the detection portion of the vibrator is certainly divided and a number of acceleration sensors are readily manufactured, thereby achieving a reduction in the cost.
In an acceleration sensor of the fourth aspect of the present invention, the vibrator has substantially a rectangular parallelepiped shape, and the groove has a depth of not less than 10% of the thickness of the vibrator. Since the groove is formed to have a depth of not less than 10% of the thickness of the vibrator in the process of dividing the detection portion of the vibrator into a plurality of parts by the formation of the groove, it is possible to achieve not only a reduction in the cost, but also an improvement in the detection sensitivity.
In an acceleration sensor of the fifth aspect of the present invention, a position where the detection portion is divided is a position where a charge distribution by the sliding vibration is substantially zero. Since the detection portion of the vibrator is divided at a position where the angular moment of the weight section is substantially zero, i.e., a position where the charge distribution by the sliding vibration of the vibrator is substantially zero, it is possible to achieve not only a reduction in the cost, but also an improvement in the detection sensitivity.
In an acceleration sensor of the sixth aspect of the present invention, the vibrator and weight section have substantially a rectangular parallelepiped shape, and a length (width) of the vibrator in an acceleration detecting direction is not more than a length (width) of the weight section in the acceleration detecting direction. It is therefore possible to reduce an error signal (crosstalk) generated when an acceleration in a direction different from that of an acceleration to be detected is applied, and to improve the S/N ratio.
In an acceleration sensor of the seventh aspect of the present invention, the weight section has substantially a rectangular parallelepiped shape, and has greater thickness and/or length (width) in an acceleration detecting direction in its portion on one side opposite to the other side connected to the vibrator than in its portion on the other side. In the weight section, since the thickness and/or the width of the portion on the side opposite to the vibrator side including the support position is made greater, it is possible to increase the angular moment of the weight section and improve the detection sensitivity.
In an acceleration sensor of the eighth aspect of the present invention, the weight section is constituted by a plurality of members of different materials. It is therefore possible to select the constituent materials of the weight section so as to increase the angular moment of the weight section and improve the detection sensitivity.
In an acceleration sensor of the ninth aspect of the present invention, the weight section has a higher density in its portion on one side opposite to the other side connected to the vibrator than in its portion on the other side. Since the weight section is constructed by using a high-dense material for the portion on the side opposite to the vibrator side, the angular moment of the weight section is increased, thereby improving the detection sensitivity.
In an acceleration sensor of the tenth aspect of the present invention, the vibrator is bonded to the weight section with an anisotropic conductive adhesive. Since the vibrator and the weight section are bonded together with the anisotropic conductive adhesive, it is possible to prevent a short circuit between the divided electrodes, provide a satisfactory connection between the vibrator and weight section, and achieve conductivity only in a desired direction.
In an acceleration sensor of the eleventh aspect of the present invention, the electrical signal is drawn from one of the divided electrodes of the vibrator, which is closer to the position of the center of gravity. Since an acceleration is not detected by a differential output of both the divided electrodes of the vibrator (a differential electrical signal of both the electrodes), but is detected by only the output (electrical signal) of one of the divided electrodes, which is closer to the position of the center of gravity, the number of patterns to be formed in the detection circuit is reduced, thereby achieving a reduction in the cost.
In an acceleration sensor of the twelfth aspect of the present invention, the thinnest portion of the weight section having a substantially rectangular parallelepiped shape is not located on one side of the position of the vibrator, including the position of the center of gravity. Since the thinnest portion of the weight section is placed on the vibrator or on a position extended from the vibrator in a direction opposite to a direction toward the position of the center of gravity, it is possible to reduce crosstalk and improve the S/N ratio.
If some object is placed in the groove of the vibrator, when an acceleration is applied, reciprocal sliding vibration of the vibrator is limited and the detection performance deteriorates. Therefore, the inside of the groove should be kept in a hollow state. Hence, in an acceleration sensor of the thirteenth aspect of the present invention, the inside of the groove is kept in a hollow state by bonding the vibrator and weight section with an anisotropic conductive film. Besides, in an acceleration sensor of the fourteenth aspect of the present invention, a protruding portion with a width of not less than the width of the groove is formed on the weight section, at a position facing the groove, and the groove is covered with the protruding portion, thereby keeping the inside of the groove in a hollow state. According to the thirteenth and fourteenth aspects, there is no possibility that an adhesive for connecting the vibrator and weight section runs into the groove, thereby easily maintaining the inside of the groove in a hollow state. As a result, reciprocal sliding vibration of the vibrator is not limited and high detection performance is obtained.
In an acceleration sensor of the fifteenth aspect of the present invention, the vibrator and weight section are bonded together with an adhesive, and the groove is filled with a filler whose Young""s modulus is smaller than that of the adhesive. If the inside of the groove is in a hollow state, certainly sliding vibration is not limited. However, the process of producing the hollow state is not easy by any means, and it is also hard to say that the process of connecting the vibrator and weight section with an adhesive while maintaining the hollow state is easy. Even when the groove is filled with some filler, if the hardness of the filler is low, sliding vibration is hardly limited. Therefore, the groove is filled with a filler whose Young""s modulus is smaller than that of an adhesive used for connecting the vibrator and weight section. In this case, it is possible to minimize the deterioration of the detection performance and readily perform the process of connecting the vibrator and weight section with an adhesive.
In an acceleration sensor of the sixteenth aspect of the present invention, the groove has a width greater than the widths of the divided detection portions of the vibrator. Since the width of the groove of the vibrator is made greater than the widths of the respective portions where the electrodes are formed, the bonded area other than the electrodes is increased, thereby improving the resistance to shock.
In an acceleration sensor of the seventeenth aspect of the present invention, the vibrator and the weight section are bonded together with an adhesive, and the area of this bonded face is greater than the area of the electrode and/or the wiring pattern. Since the area of the electrode and/or the wiring pattern is made smaller than the bonded area between the vibrator and weight section, even when the electrode and/or the wiring pattern do not easily stick to the adhesive, the vibrator and weight section can be bonded at portions other than the electrode and wiring pattern, thereby achieving a high bonding strength.
In an acceleration sensor of the eighteenth aspect of the present invention, the weight section has a cavity for storing a part of the vibrator. Since a region of the weight section connected to the vibrator has a cavity structure, even when its portion connected to the vibrator is made thinner so as to improve the detection sensitivity, the weight section is not twisted by an applied acceleration, thereby limiting an increase of crosstalk and improving the resistance to shock.
In an acceleration sensor of the nineteenth aspect of the present invention, the vibrator is a single crystal piezoelectric element. Since a single crystal piezoelectric element is used for the vibrator, it is possible to reduce crosstalk and improve the S/N ratio.
In an acceleration sensor of the twentieth aspect of the present invention, the single crystal piezoelectric element is an X-cut plate of lithium niobate (LiNbO3). Since the X-cut plate of LiNbO3 which does not have an electromechanical bonding in a thickness direction is used as the vibrator, it is possible to reduce crosstalk and improve the S/N ratio.
An acceleration sensor device of the twenty first aspect of the present invention comprises an acceleration sensor as described above; and a package, including a base provided with a wiring pattern and a cap covering the base, for storing the acceleration sensor therein. Since the acceleration sensor is stored in the package including the base provided with the wiring pattern and the cap, it is possible to readily perform the process of storing the acceleration sensor and to easily construct a circuit for drawing an electrical signal.
An acceleration sensor device of the twenty second aspect of the present invention includes in a package a detection circuit for detecting an acceleration based on an output electrical signal. Since the detection circuit is stored in the package together with an acceleration sensor, it is possible to improve the S/N ratio.
In an acceleration sensor device of the twenty third aspect of the present invention, a part of the weight section is sandwiched by package. It is therefore possible to readily provide an electrical connection between the weight section and package and to reduce the size of the device.
In an acceleration sensor device of the twenty fourth aspect of the present invention, a part of the weight section and a part of the vibrator are sandwiched by the package. It is therefore possible to readily provide an electrical connection between the weight section and package and to reduce the size of the device.
In an acceleration sensor device of the twenty fifth aspect of the present invention, the part of the weight section sandwiched by the package is a portion located on one side of the position of the vibrator opposite to the other side including the position of the center of gravity. Since the vibrator and the portion of the weight section located on one side of the position of the vibrator opposite to the other side, including the position of the center of gravity are sandwiched by the package, even though they are sandwiched by the package, the sandwiched portions are apart from the position of the center of gravity on which an acceleration applies. Therefore, the detection sensitivity is not lowered.
In an acceleration sensor device of the twenty sixth aspect of the present invention, the length of the part of the weight section sandwiched by the package is not more than the length of the vibrator. It is therefore possible to improve the detection sensitivity.
In an acceleration sensor device of the twenty seventh aspect of the present invention, the vibrator has a package-side electrode, the package is provided with a wiring pattern connected to the electrode, the vibrator and package are bonded together with an adhesive, and the area of the bonded face is larger than the area of the electrode and/or the wiring pattern. Therefore, even when the electrode and/or the wiring pattern do not easily stick to the adhesive, the vibrator and package are bonded together at portions other than the electrode and wiring pattern, thereby achieving a high bonding strength.
A method of fabricating an acceleration sensor of the twenty eighth aspect of the present invention comprises the steps of forming a groove in a vibrator; applying a material produced by dissolving a powder in a volatile solvent to the groove; evaporating the volatile solvent; and connecting the vibrator and a weight section with an adhesive. The powder layer functions as a cap to prevent the adhesive from entering the groove, thereby maintaining the inside of the groove in a hollow state.
A method of fabricating an acceleration sensor of the twenty ninth aspect of the present invention comprises the steps of forming a groove in a vibrator; inserting a structural body into the groove; connecting the vibrator and weight section with an adhesive; and removing the structural body. This method can prevent the adhesive from entering the groove and maintain the groove in a hollow state.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.